Effect of spin-orbit interaction on the electronic and optical properties of ultrathin bismuth nanowires - a density functional approach

A first-principles study of the effects of spin -orbit coupling (SOI) on the structural, electronic and optical properties of 16 bismuth nanowires, Bin with n = 7 -18, has been performed. The density functional theory (DFT) in the local density approximation (LDA) has been used. The inclusion of the SOI significantly alters the electronic and optical properties of the wires. The stable structures for the Bin wires with n = 7 -18 form two groups: non-helical and helical configurations. In addition to the most stable non-helical 5-Bi pentagonal, 6-Bi hexagonal and 6-Bi triple-zigzag wire configurations found in a previous report, the present study adds to this list three more non-helical structures, namely the non-helical 7-Bi hexagonal, 8-Bi heptagonal and 11-Bi pentagonal cross-sectional wire configurations. The present result is in sharp contrast to the conclusions of our previous studies of Pb-and Tl-nanowires, where it was observed that, in general, a structure possessing a high coordination number value has large binding energy and, therefore, the helical wire structures are the most stable ones. All of the wires are metallic in the LDA. The number of channels in the nanowires is large which will lead to high quantum ballistic conduction. The consideration of the many body effects such as the GW approximation (GWA) may destroy the metallicity predicted here in the Bin wire configurations for n <= 7. However, for the wire configurations having n >= 8, we find that even in the GWA, one may not observe the opening of the energy gaps because of the violation of the electron counting principle. The optical absorption calculated with SOI is much stronger compared to the one obtained after neglecting the SOI. For the wires containing a large number of atoms in the unit cell, the optical absorption is multi-peaked, strong and extended over the whole energy region from infrared to the ultraviolet electromagnetic radiation including the visible region. These nanowires may thus be used as a source of white radiation.